The present invention relates to a reference current source for providing a current that is at least approximately temperature-independent within a temperature interval.
A known circuit for generating a temperature-independent current has a bandgap reference, as is described for example in Tietze, Schenk: xe2x80x9cHalbleiterschaltungstechnikxe2x80x9d [Semiconductor circuitry], Springer Verlag, Berlin, 1991, page 558, and a largely temperature-stable resistor. In this case, the resistor is connected to an output of the bandgap reference at which a temperature-independent output voltage is present, and a temperature-independent current flows through said resistor, which current can be fed to an application circuit via a simple current mirror circuit.
Problems can arise through the use of a bandgap reference and a resistor for reference current generation in integrated circuits using CMOS technology. In CMOS technology, resistors can be fabricated with the required accuracy only with very great difficulty. Moreover, the resistances of such resistors are greatly dependent on temperature.
U.S. Pat. No. 4,843,265 discloses using a MOS transistor for generating a reference current. For compensation of a temperature dependence of the drain-source current of a MOS transistor, in the known reference current source a circuit arrangement is connected to the gate terminal, which circuit arrangement generates a control voltage which is dependent on absolute temperature and counteracts the temperature drift of the drain-source current.
An approach similar to that in U.S. Pat. No. 4,843,265 is pursued in the case of a known reference current source according to Blauschild: xe2x80x9cAn Integrated Time Referencexe2x80x9d, 1994, International Solid State Circuits Conference, Paper WP3.5.
In the case of the current source both according to U.S. Pat. No. 4,843,265 and according to Blauschild, good bipolar transistors are necessary in order to generate a drive voltage which counteracts the temperature drift of the drain current. Although parasitic bipolar transistors are available in all bulk CMOS processes, their electrical properties allow reproducibility to an ever poorer extent in CMOS processes, particularly in the xe2x80x9cDeep-Submicronxe2x80x9d range.
It is an aim of the present invention to provide a reference current source which supplies an at least approximately constant current within a temperature interval and which can be realized simply and cost-effectively using CMOS technology.
The reference current source according to the invention has a first voltage-controlled current source having at least one first control voltage source for providing a first temperature-dependent control voltage and having at least one first MOS transistor. In this case, a first current is available at an output of the first voltage-controlled current source, which current is dependent on the control voltage and a process gain of the at least one first MOS transistor. The reference current source furthermore has a second voltage-controlled current source having at least one second control voltage source for providing a second control voltage and having at least one second MOS transistor. In this case, a second current is available at an output of the second voltage-controlled current source, which current is dependent on the second control voltage and a process gain of the at least one second MOS transistor. Furthermore, an addition unit is provided for the purpose of forming a reference current from the first and second currents of the first and second current sources. The process gain K of a MOS transistor results, as is known, from the product of the temperature-dependent charge carrier mobility xcexc and a capacitance per unit length Cox, which is dependent, inter alia, on the thickness of the gate oxide. In the case of the reference current source according to the invention, in which the first current is dependent on the temperature-dependent first control voltage and the temperature-dependent process gain K, and in which the second current is dependent on the temperature-dependent process gain and the second control voltage, the first and second currents can be set by means of suitable dimensioning of the MOS transistors in the current sources or by means of suitable weighting of the currents prior to their addition in such a way that the reference current resulting from the first and second currents is at least approximately temperature-independent within a temperature interval.
The first control voltage, which is dependent on temperature and is preferably proportional to absolute temperature, can be generated with sufficient accuracy by a bipolar transistor, in particular by a parasitic bipolar transistor present in every bulk CMOS circuit.
The second control voltage is configured in particular in such a way that the derivative of the first control voltage with respect to temperature and the derivative of the second control voltage with respect to temperature are not identical. The second control voltage is preferably constant within the relevant temperature interval within which the reference current is intended to be constant, or, within this interval, is inversely proportional to absolute temperature.
The current supplied by the first and second voltage-controlled current sources preferably satisfies the following relationship:
Ixe2x88x9dKxc2x7Uc2xe2x80x83xe2x80x83(1)
where I designates the respective output current of the first or second current source and Uc designates the respective control voltage.
W. M. Sansen et al.: xe2x80x9cA CMOS Temperature-Compensated Current Referencexe2x80x9d, IEEE Journal of Solid State Circuits, vol. 23, No. 3, June 1988, describe the basic construction of an exemplary embodiment of a current source whose output current satisfies the relationship (1). The circuit arrangement essentially has two MOS transistors whose control terminals are coupled by means of a control voltage source and through which the current I flows in each case.
A proportionality factor A not contained in equation (1) is dependent on the dimensioning of the two MOS transistors in each voltage-controlled current source. Mathematically, it can be shown that the output currents of the first and second voltage-controlled current sources can be weighted by means of suitable dimensioning of the two MOS transistors or by means of multiplication of the output currents by suitable weighting factors prior to addition in such a way that the reference current is at least approximately temperature-independent.